This invention relates to asymmetric digital subscriber lines (ADSL), and more particularly to a method and apparatus for interfacing ADSL modems to a codec.
ADSL modems are used to achieve a very high speed of data communication. They employ discrete niultitone (DMT) technology to communicate large amounts of data, such as are required for transmitting digital television broadcast, on-demand video, teleconferencing and home shopping applications over conventional twisted paired telephone lines. Unlike traditional data modems, ADSL modems require a corresponding modem to be placed in an access module of a telephone company""s central office (CO ADSL).
The splitterless ADSL transceiver described in ITU-T Draft G.992.2 (G.992.2) is an interoperable ADSL modem (G.992.2 ADSL modem). Such modems are designed based on certain modifications that reduce the standard functions of heavy ADSL modems designed according to T1.413 Issue 2 specifications. In particular, the frame size of each symbol is reduced from 512 samples or 544 samples with cyclic prefix to 256 samples or 272 samples with cyclic prefix. Further, one hundred twenty eight (128) tones are used instead of two hundred fifty-six (256), and the receiving and transmitting sampling rates are 1.104 MHz instead of 2.208 MHz.
FIG. 1 shows a conventional interface 10 for a G.992.2 modem and a codec 11. The interface uses a total of five frame buffers, two frame buffers 12 and 13 for transmitting data to codec 11 and three frame buffers 14, 15 and 16 for receiving data from codec 11. Specifically, separate frame buffers are used for transmitting circuitry 17 and receiving circuitry 18.
Conventional interfaces of the type shown in FIG. 1 suffer from several significant drawbacks. Specifically, they require a large number of buffers for interfacing with a codec. For 272 samples with cyclic prefix, a conventional G.992.2 ADSL modem uses 1360 words of data memory to interface with a codec. In addition, such conventional interfaces are not easily interoperable with heavy ADSL modems. Finally, they are not easily extendable to implement a heavy ADSL modem because of the additional memory required which adds to the complexity and cost of the device.